A number of catalysts are known to be effective for the oxidation of acrolein or methacrolein to acrylic acid or methacrylic acid, respectively. However, the yields obtained using the catalysts for the preparation of methacrylic acid are low. West German Provisional Patent No. 2,048,620 discloses catalysts containing the oxides of molybdenum, phosphorus, and arsenic for the oxidation of methacrolein and acrolein to methacrylic acid and acrylic acid, respectively. U.S. Pat. No. 3,761,516 discloses catalysts containing oxides of molybdenum, arsenic and phosphorus on a support, especially Al.sub.2 O.sub.3, having external macropores and a surface not greater than 2 m.sup.2 /g.
The present invention is a result of a search for more efficient and desirable catalysts for the production of acrylic acid and methacrylic acid. Unexpectedly higher yields of and selectivities to acrylic acid and methacrylic acid are obtained by the vapor phase oxidation of acrolein and methacrolein, respectively, with molecular oxygen in the presence of the new and useful catalysts of the present invention.